[ntp:questions] Time going backwards

Steve Kostecke kostecke at ntp.org
Thu Aug 14 16:07:49 UTC 2008

On 2008-08-13, John Bartas <jbartas at speakeasy.net> wrote:
> Hi Folks,
> I have some questions about when (if ever) NTP will set time backward. 
> I'll be using it on Embedded Linux for time stamping video frames.
> I've found several mentions in RFC 1305 that this should not happen, and 
> also in section "Step Phase Adjustments" mention that it can jump 
> backwards.

ntpd has a default step threshold of 128ms. When the offset does not
exceed this threshold the clock is slewed. When the offset exceeds this
threshold the clock is stepped.

> Is there a simple description anywhere if what variables affect this, 
> and how? I'd like to know when it's going to jump backwards on Linux - 
> what the rules are and what control I have over it.

The ntpd manual page,
http://www.eecis.udel.edu/~mills/ntp/html/ntpd.html (dev release)
or http://doc.ntp.org/4.2.4/ntpd.html (stable release), provides a
description of now NTP works. Start at the "How NTP Operates" section.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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